Hydrous alcohol distillation method and apparatus

Abstract

An improved distillation method and apparatus are provided for recovering hydrous ethanol from fermentation or synthetic feedstocks. Substantial energy savings are realized by utilizing a pair of stripper-rectifier towers in which overhead vapors from one tower operating at a higher pressure supply the heat required for the other tower operating at a lower pressure and by preheating the feedstock in multiple heat exchange steps. The feedstock is split into two portions of unequal size, the larger portion being supplied to the higher pressure tower and the smaller portion to the lower pressure tower.

We claim: 
     
       1. A distillation method for recovering hydrous ethanol from a dilute ethanol-containing feedstock which comprises: (a) partially preheating a feedstock stream;   (b) splitting the partially preheated feedstock stream into two feed streams of unequal size;   (c) further preheating both the larger and the smaller of said feed streams;   (d) introducing the preheated feed streams in parallel into a pair of stripper-rectifier towers;   (e) maintaining a higher pressure in the stripper-rectifier tower receiving the larger feed stream than in the stripper-rectifier tower receiving the smaller feed stream;   (f) condensing steam to supply the heat required in the higher pressure stripper-rectifier tower;   (g) condensing overhead vapors from the higher pressure stripper-rectifier tower to supply the heat required in the lower pressure stripper-rectifier tower, and returning the resultant condensate as reflux to said higher pressure stripper-rectifier tower;   (h) condensing overhead vapors from said lower pressure stripper-rectifier tower, and returning the resultant condensate as reflux to said lower pressure stripper-rectifier tower;   (i) withdrawing a hydrous ethanol product stream from the upper portion of said higher pressure stripper-rectifier tower, and cooling the same;   (j) withdrawing a hydrous ethanol product stream from the upper portion of said lower pressure stripper-rectifier tower;   (k) combining the cooled hydrous ethanol product stream from step (i) with the hydrous ethanol product stream from step (j);   (l) cooling the combined hydrous ethanol product stream; and   (m) withdrawing a bottoms stream from each of said stripper-rectifier towers and separately cooling the same;   (n) said partial preheating of said feedstock stream being effected in stages by means of the heat obtained in cooling step (1), in condensing step (h), and by heat exchange with stream condensate from condensing step (f);   (o) said further preheating of said smaller feed stream being effected by means of the heat obtained in step (m) during cooling of the bottoms stream from said lower pressure stripper-rectifier tower; and   (p) said further preheating of said larger feed stream being effected in stages by means of heat obtained in cooling step (i) and in step (m) during cooling of the bottoms stream from said higher pressure stripper-rectifier tower.   
     
     
       2. The method of claim 1 wherein the larger of said feed streams comprises more than 50 wt.% but not more than about 70 wt.% of the total feed, and the smaller of said feed streams comprises less than 50 wt.% but not less than about 30 wt.% of the total feed. 
     
     
       3. The method of claim 1 wherein the condensate streams in steps (g) and (h) are returned as reflux to the respective towers, except for the removal of a heads draw from each of said condensate streams to avoid accumulation of low-boiling impurities in the combined hydrous ethanol product stream. 
     
     
       4. The method of claim 1 wherein at least one fusel oil side draw containing higher boiling impurities is removed from an intermediate portion of each of said stripper-rectifier towers, said fusel oil side draws are combined and washed with water, a fusel oil layer is separated from an aqueous ethanol-containing layer, and said aqueous ethanol-containing layer is returned to said lower pressure stripper-rectifier tower. 
     
     
       5. The method of claim 4 wherein said fusel oil layer is blended with said combined hydrous ethanol product stream. 
     
     
       6. The method of claim 4 wherein the hydrous ethanol product stream is withdrawn from the corresponding stripper-rectifier tower at a location below the point of return of said reflux but above the point of removal of said fusel oil side draw. 
     
     
       7. The method of claim 1 wherein the pressure in said higher pressure stripper-rectifier tower is from atmospheric pressure to about 100 psig and the pressure in said lower pressure stripper-rectifier tower is from sub-atmospheric to about 50 psig. 
     
     
       8. The method of claim 7 wherein the pressure in said higher pressure stripper-rectifier tower is about 50 psig and the pressure in said lower pressure stripper-rectifier tower is about atmospheric pressure. 
     
     
       9. The method of claim 1 wherein at least said higher pressure stripper-rectifier tower contains a plurality of vertically spaced baffles comprising substantially smooth surfaced imperforate plate members which are free of weirs, seals, bubble caps, downcomers and the like, said baffles being arranged to provide transversely offset open areas so that ascending gas or vapor phase traverses a tortuous path upwardly through the tower as liquid phase descends therethrough; and the flow rate of said gas or vapor phase is correlated with the vertical spacing between adjacent baffles and with the open areas of said baffles such that the horizontal velocity factor f h  between adjacent baffles is within the range of from 0.20 to 0.80 feet per second and the vertical velocity factor f v  in the open areas of said baffles is substantially less than the horizontal velocity factor f h  and is within the range of from 0.10 to 0.40 feet per second, said velocity factors being determined in accordance with the following equations: ##EQU2##  where U v  is the velocity in feet per second of the gas or vapor phase passing through the open areas of said baffles, U h  is the velocity in feet per second of the gas or vapor phase passing through the descending liquid phase between adjacent baffles, ρ v  is the density in pounds per cubic foot of the ascending gas or vapor phase at the temperature and pressure in the tower, and ρ l  is the density in pounds per cubic foot of the descending liquid phase at the temperature in the tower;   whereby the liquid phase overflows the baffle edges as a continuous curtain and is dispersed into discrete droplets or froth by high velocity gas or vapor phase in the vertical spaces between adjacent baffles and a decreased gas or vapor phase velocity is obtained in the open areas of the baffles permitting the major part of the liquid droplets or froth to coalesce and separate from the ascending gas or vapor phase.   
     
     
       10. In the method of recovering hydrous ethanol from a dilute ethanol-containing feedstock wherein said feedstock is split into two feed streams, said feed streams are introduced in parallel into a pair of stripper-rectifier towers, a higher pressure is maintained in one of said towers than in the other, said one tower is heated by condensation of steam in the reboiler of said one tower, overhead vapors from said one tower are condensed to supply the heat required in said other tower, and a hydrous ethanol product stream and a bottoms stream are removed from each of said towers; the improvement which comprises: (a) partially preheating said feedstock in stages by means of the heat obtained by cooling the combined hydrous ethanol product streams, by condensation of the overhead vapors from said other tower, and by heat exchange with steam condensate from said reboiler of said one tower;   (b) further preheating the feed stream being introduced into said other tower by means of heat obtained by cooling of the bottoms stream from said other tower; and   (c) further preheating the feed stream being introduced into said one tower by means of heat obtained by cooling the hydrous ethanol product stream from said one tower, and by cooling of the bottoms stream from said one tower.   
     
     
       11. The method of claim 10 wherein the feed stream introduced into said one tower comprises more than 50 wt.% but not more than about 70 wt.% of the total feed, the feed stream introduced into said other tower comprises less than 50 wt.% but not less than about 30 wt.% of the total feed, the pressure in said one tower is from atmospheric pressure to about 100 psig, and the pressure in said other tower is from sub-atmospheric to about 50 psig. 
     
     
       12. A distillation apparatus for recovering hydrous ethanol from a dilute ethanol-containing feedstock comprising, in combination: means for partially preheating a dilute ethanol-containing feedstock stream; means for splitting the partially preheated feedstock stream into two feed streams of unequal size; means for further preheating both the larger and the smaller of said feed streams; a higher pressure stripper-rectifier tower; a lower pressure stripper-rectifier tower; means for introducing the larger of said feed streams into said higher pressure tower; means for introducing the smaller of said feed streams into said lower pressure tower; first reboiler means for condensing steam to supply the heat required in said higher pressure tower; steam condensate cooler means for cooling the steam condensate from said first reboiler means; second reboiler means for condensing overhead vapors from said higher pressure tower to supply the heat required in said lower pressure tower; means for returning the resultant overhead vapor condensate as reflux to said higher pressure tower; condenser means for condensing overhead vapors from said lower pressure tower; means for returning the resultant overhead vapor condensate as reflux to said lower pressure tower; means for withdrawing a hydrous ethanol product stream from an upper portion of said higher pressure tower; first product cooler means for cooling said hydrous ethanol product stream from said higher pressure tower; means for withdrawing a hydrous ethanol product stream from the upper portion of said lower pressure tower; means for combining the cooled hydrous ethanol product stream from said first cooler means with the hydrous ethanol product stream from said lower pressure tower; second product cooler means for cooling the combined hydrous ethanol product streams; means for withdrawing a bottoms stream from each of said towers; and separate bottoms cooler means for cooling the bottoms stream from each of said towers; said means for partially preheating said feedstock stream comprising means for passing said feedstock in heat exchange relation through said second product cooler means and then through said condenser means and then through said steam condensate cooler means; said means for further preheating said smaller feed stream comprising means for passing said smaller feed stream in heat exchange relation through said bottoms cooler means for cooling the bottoms stream from said lower pressure tower; and said means for further preheating said larger feed stream comprising means for passing said larger feed stream in heat exchange relation through said first product cooler means and then through said bottoms cooler means for cooling the bottoms stream from said higher pressure tower.